In aircraft, there is now a move to greater use of electrical power and electronic systems rather than hydraulic and pneumatic systems as this has the potential of leading to lighter aircraft. This trend to more electric aircraft (MEA) means that modern aircraft include large generators that generate more electrical power during flight than today and this generated power is used to supply power to more and different on-board aircraft electric power systems. The generators use rotation of the engine to generate AC power using known techniques. The electric frequency on more recent aircrafts may range from 350-800 Hz, while the AC voltage is usually regulated at a fixed value such as 115 Vac or 230 Vac. While the aircraft engines are not running, the on-board electric systems are generally powered by ac power from a ground cart. Such power is typically 115V/230V 400 Hz AC power.
While aircraft electrical power is generated in the form of three-phase alternating current, most electrical loads require DC power to operate, and so conversion of AC to DC power is required. The direct rectification of 3-phase AC power into DC power is simple and straightforward, e.g. using 3 pairs of diodes, however this creates unacceptable levels of current distortion or harmonics. These distortions cause power quality issues which are difficult to address, especially since, in modern aircraft, the fundamental frequency may vary over a wide range. Regulating bodies have imposed stringent power quality requirements including limitations on harmonic currents that can be drawn from an aircraft ac power system.
Multipulse power conversion is one of several technologies capable of AC-DC power conversion with low distortion levels that meet aerospace power quality standards.
A typical autotransformer-based multipulse converter contains two major functional blocks—a multipulse autotransformer and a rectifier. Autotransformer rectifier units (ATRU) have a low part count and are highly reliable. They have only few low frequency switching components, so EMI emissions are low. However the ATRU adds considerable weight and cost to an aircraft system equipped that way.
A multiphase converter performs a phase shifting process through transformers to convert from an original three-phase ac supply to multiphase ac supply to result in a higher number of pulses in dc output to result in a close to sinusoidal current draw with reduced harmonic distortion at the ATRU input.
An n-pulse ATRU is composed of n/6 6-pulse diode bridges (n/2 diode pairs) and uses phase-shifting of the secondary voltages in the autotransformer. A three phase fixed or variable frequency—constant amplitude voltage source supplies power to the ATRU, thus providing three different phase shifted sinusoidal 3 phase voltages
ATRUs used in aircraft are typically 18-pulse converters, but others, e.g. 12-pulse, 24-pulse, etc. may be used. A 12-pulse ATRU, for example, is described in “Polyphase Transformer Arrangements with Reduced kVA Capacities for Harmonic Current Reduction in Rectifier-Type Utility Interface”, S. Choi, P. N. Enjeti and I. J. Pitel. IEEE Transactions on Power Electronics, vol. 11, pp 680-690, 1996. An overview of multipulse AC-DC converters can be found in “Multipulse AC-DC Converters for Improving Power Quality: A Review”, B. Singh, S. Gairola, B. N. Singh, A. Chandra; K. Al-Haddad. IEEE Transactions on Power Electronics, vol. 23, No. 1, January 2008, pp. 260 ff.
FIG. 1 shows a block diagram of a typical 18-pulse rectifier unit including the Autotransformer Rectifier Unit (ATRU), diode bridges and Interphase Transformers (IPTs). This shows the ATRU 1 comprising an isolated or non-isolated multi-winding transformer that displaces the AC input voltage a number of degrees dependent on the number of secondary windings.
The output voltage of the multi-phase winding is rectified by a number of basic 6-pulse rectifier cells 2, thus improving the input current total harmonic distortion. The input current quality, however, is limited by the number of 6-pulse stages. If additional 6-pulse cells are added to achieve a lower distortion, this results in an undesirable increase in the weight of the rectifier unit.
The aim of the present disclosure is to provide a power converter that mitigates harmonic distortion and adds redundancy to the rectification stage with either fixed frequency or variable frequency AC power source, while avoiding the use of the autotransformer unit and the IPT, thus reducing the rectifier unit weight compared with the existing ATRU solution.